The Ultimate Ally
by AgentVenom8190
Summary: Ben Tennyson, Wielder of the Ultimatrix, Savior of the Universe, had done alot since gaining the Omnitrixand later, the Ultimatrix, but saving the universe from a race of machines hell-bent on genocide? Yeah, that's a new one for our favorite shapeshifting hero. [Ben X Tali'Zorah]
1. Chapter 1: New universe, new villains

**A/N: Hey guys! Sorry i was on vacation with my family and my story 'A Venomous Ally' will be put on hold till I regain muse to write more onto it! Anywho! This is a Ben 10 and Mass Effect crossover adopted from Darth Terminus! I loved the work they did on their Ben 10 and Mass Effect story so I'm gonna give it a shot! And yes, Ben will have the Ultimatrix because who doesn't wanna see Ben blow stuff up as Ultimate Humgosaur? This will take place through Classic and Omniverse series, though some Reboot aliens*cough*Shock Rock and Gax*cough* will make an appearance!**

**Bold: Yelling**

_Italicized: Thoughts_

**_Bold and Italicized: Alien Database_**

**Bold and Underlined: Location**

**\- Bellwood, 2:45 P.M, year 2017 -**

Seven years ago, people barely knew Bellwood existed, but one summer changed that forever, a ten year old boy had accidentally found a watch that contained DNA from ten different species of aliens, which expanded as he grew older, now, Bellwood was practically a hotspot due to all the alien activity.

Now, seventeen year old Ben Tennyson, Wielder of The Ultimatrix and Savior of the Universe, was at his favorite place in the universe(s), Mr. Smoothies, sipping on a smoothie and looking at the Ultimatrix, Azmuth finally grew tired of Albedo's crap and took the Ultimatrix away from him, breaking it down till he came across the Polymorphic Crystal in the device, now, while Azmuth would never admit it, he knew Albedo was a genius when it came to wepon designing.

Azmuth believed Ben was mature enough to handle the Evolutionary Function of the Ultimatrix so he placed it in Ben's Omnitrix, causing tit to change into the Ultimatrix...And he knew that Paradox was gonna send Ben to another universe, Paradox showed Azmuth that universe...a race of machines hell-bent on genocide? Thtat didn't bode well with him...So he knew Ben would need all the help he could possibly get.

Ben smiled at the Ultimatrix, he had missed the Evolutionary Function alot, mainly because nownow he knew that all aliens could have a Ultimate form...but he'll only use them when the situation calls for it. He then got up and threw the cup in the trash, hearing the familiar sound of a portal opening behind him, he turned and smiled at the blue portal as a man with a 1960's getup walked out, a smile plastered on his face.

"Hello young Benjamin!" The British time-walker man called out in a joyful tone, the teen smiled at the older man.

"Hey Paradox, while its good to see you, I'm guessing you aren't here on a social call?" Ben quipped, but once a grim expression came across Paradox's normally jovial features, he knew something was up.

"I'm afraid not young Benjamin, I have been traveling across the vast multiverse and I have come across one that truly disturbs me...in this universe, a race of sentient machines called the 'Reapers' wipe out almost all other sentient life every fifty thousand years..." Paradox said, and Ben had the sudden urge to throw up now, almost complete extinction of every race? Yeah, no way was he gonna allow that to continue.

"Say no more, just point me in the right direction and I'll make sure these 'Reapers' get a taste of their own medicine." He smashed his fist into his other hand, which was open. Paradox smiled at the boy's enthusiasm and attitude, always willing to help others out, no matter the situation.

"Splinded! But I must warn you, you won't be able to return until a very long time, but I have already taken the liberty of telling your parents, Azmuth, your cousin and Kevin, and your Grandfather, Max." Ben froze, not being able to return? He shook his head, knowing that even if he never returned, they'd be proud of what he was doing, so he nodded and grinned.

"Let's do it, but I have to grab something first." Paradox nodded and transported them to Ben's parent's house, where everyone was currently, and if Ben was gonna save a new universe...he was gonna do it in stylestyle. Ben walked into the home, looking at everyone and he gave them a small smile. "Hey guys...I'm off to save a different universe." He said and Kevin, Ben's enemy turned best friend laughed and got up, patting Ben on the shoulder.

"Sounds like a normal Monday to me. Stay safe knucklehead." Kevin said, causing Ben to laugh and he grinned. "You too, doofus." He then turned to his cousin, Gwen, who just smiled and hugged him, a hug whice he happily returned.

"Be careful Ben." She said with a smile, Ben chuckled softly.

"Aren't I always?" He said, his parents didn't say anything just hugging him anand smiling, Ben returned the hug before looking at his Grandpa Max, who smiled and in his hands was a green leather jacket with a strip of white on the left side, housing a black '10' in the white circle, located above where the heart would be. He hugged his grandson and smiled.

"Make the Tennyson name proud Ben." He then hended the jacket to Ben which he accepted and put on.

"Don't I always?" He said, then a very clear 'Ahem' resonated from the rom, Ben turned around only to see his favorite Galvan, Azmuth.

"Well you just gonna stand there al day or are you gonna come here?" The impatient Galvan said in a somewhat joking matter, Ben walked to the frog like alien and Azmuth jumped onto the Ultimatrix, twisting the dial a few times before looking at Ben. "I've improved the timer functionality so you can stay in alien forms much longer and I've also unlocked a few new alien forms for you Tennyson...Go make us all proud." Azmuth said and he jumped off Ben's hand and right into MaMax's and Ben smiled and nodded, he walked to Paradox, who had a portal open and ready, he gave one last looked at his friends and family before he smiled.

"Goodbye guys, try not to break everything while I'm gone." He walked through the portal with Paradox and he looked at the time-walker. "Any advice before I go?"

Paradox hummed and smiled. "There is a species of robots and insectoids helping the Reapers, mainly the Collectors and Geth, do not hesitate to put the Collectors out of their misery, you'd be doing them a favor, and look for a man who goes by Commander Shepherd, him and his team are trying to stop the Reapers, and do be patient with Miss Lawson...Gumball?" He offered one to Ben, who normally turned them down but this time, he nodded.

"Yeah...I'll take one." He smiled and pulled out a green one and placed it into his pocket, Paradox smiled before disappearing and leaving Ben to free fall downwards towards the ground.

**\- Horizon, 9:52 A.M, year 2185 -**

"PARADOX!!!" He yelled before quickly activating the Ultimatrix, spinning the dial before landing on and old friend, he slammed the dial down and he felt his DNA chaging, in a flash of green emerald light, the boy was replaced by a creature that was black with green going down its center torso, two antennae with plugs on the end, and at tail with the same, his fingers had plugs as well and he had one eye and the Ultimatrix symbol rested on his chest.

**"Feedback!" **The transformed teen called out, shooting electricity from his fingers to slow his fall.

**Alien Database.**

**Name: Feedback**

**Species: Conductoid**

**Homeplanet: Teslavorr**

**Abilities: The species, Conductoids, are well known for being able to absorb and redirect electricity, but they also have increased stamina and strength, but they are not invulnerable, if a Conductoid doesn't absorb electricity, it's own reserves of bio-electricity will quickly bee drained.**

Ben landed on the ground, looking up he saw that he gained the attention of a few Collectors. He ququickly went to work and zapped a few of them, killing them and his eye widened. _'Wow, Paradox was right.' _He thought to himself before a mssive Geth cane forward.

**"WE ARE HARBINGER." **It called out, Ben turned around and adopted a fighting stance.

"Good for you." Ben said before slapping the Ultimatrix dial and switching forms, now in place of the Conductoid, a massive alien made of diamond stood.

**"Diamondhead!" **The new alien called out and began beating Harbinger to scrap metal.

**Alien Database.**

**Name: Diamondhead**

**Species: Petrosapian**

**Homewolrd: Petropia**

**Abilities: Petrosapians can control their crystal physiology at will, forming curde weapons or very sharp blades, they are also capable of shooting these crystals, but they aren't invincible, enough force and they willl shatter and their main weaknesses is sonic attacks.**

**"SHAPESHIFTING CAPABILITIES; YOU ARE AN UNKNOWN, YOU WILL BE DESTROYED."** Harbinger said whilwhile Diamondd head looked unamused.

"If I had a nickel for everytime I've heard that..." He muttered before he noticed Harbinger's lights flicker.

**"ASSUMING DIRECT CONTROL." **It said before finally dying and Diamondhead stood up and looked around.

"Now I have to find Shepherd...I just follow the gunshots then." The Petrosapian said before running in the direction of the gunfire.

**A/N: Wow! This was a lot of fun to write! I hope y'all read this and enjoy it! I do not own Ben 10 or Mass Effect! Btw, incase you havent guessed it, this is set during ME2!**


	2. Introductions, more or less

**A/N: Wow, I honestly didn't think this story would get that much attention, and to user EntertainmentChampion for sending me that awesome review, this one is for you man! And I have decided that y'all can pick what aliens Ben keeps to himself from Shep and the gang, aliens like Alien X, Atomix, and Clockwork are already gonna be under tight security as those aliens are his most powerful ones, just use the review system to tell me which aliens should be kept hidden! And which new Ultimate aliens I should bring in! And aliens like Waltatrout and The Worst will not be in this story.**

Ben as the bulky figure of Diamondhead when he stopped to catch his breath, that's when the realization hit him, making him feel very stupid. "Why am I running as Diamondhead?!" He yelled at himself as he slapped the Ultimatrix symbol, his form becoming a lot less bulky, sporting a tail with blue strips and balls on his feet.

**"XLR8!" **The velociraptor looking alien called out in a raspy voice before speeding off in a black and blue blur towards the gunfire.

**Alien Database.**

**Name: XLR8**

**Species: Kineceleran**

**Homeworld: Kinet**

**Ablities: Kinecelerans are literal speed demons, able to run at speeds that could very well break the sound barrier if being pushed tho their limits, claws on their hands can be used as a deadly weapon at such speeds. XLR8 is one of Ben Tennyson's original ten aliens, and a fan favorite.**

The transformed teen arroved in time to see Shepard's crew, composed of a bird-like alien, four other humans, one male and three females, Commander Shepard himself, a dinosaur looking alien, and a brow lizard looking one. "I hope thats Shepherd and his crew, time to give them a hand." And with that, he took off like a bullet exited that chamber, Shepard and his crew barely had time to comprehend what was going on as all the Collector's thorats were slit, causing them the fall dead to the ground.

"What the hell?.." The bald tattoo headed woman said before everyone saw the velociraptor looking alien stop in front of them, the alien looked at Shepard.

"You Commander Shepard?" XLR8 asked, his helmet's faceplaye opened up to reveal his face, after quickly regaining his composure, he gave a nod but kept his gun trained on Ben, as did the others.

"Yeah, depends on who's asking." Shepard asked with caution but Ben only nodded.

"Name's XLR8, for now, I was sent here to helphelp you." Ben said before Shepard motioned towards the AA Cannons, EDI had been trying for a few minutes to get them operational.

"Yeah, you good with tech? We need those AA Cannons to secure this area." He explained to the unknown alien, who shook his head.

"No, but I got something that is, and it will surprise you." The transformed teen said with a slight smirk.

"Honestly, I've seen weirder shit, so nothing can really surprise me." He said, wondering what this alien would do.

"Hundred bucks says you're wrong." XLR8 said somewhat smugly to the Commander.

_'Bucks? That term hasn't been used since the 21st century...but I'll bite.' _Shepard then smirked and lowered his fighting stance. "You're on."

Shepard and everyone else watched at the alien touched the hourglass symbol on its chest, and in a green flash the Kinerceleran was relplaced by a alien with and black and green tech patten design, the strange symbol now placed where its eye would be.

**"Upgrade!" **The Galvanic Mechamorph called out, looking at everyone.

**Alien Database**

**Name: Upgrade**

**Species: Galvanic Mechamorph**

**Homeworld: Galvan B**

**Abilities: Galvanic Mechamorphs are a race artificially created by First Thinker, Azmuth, they can merge awith tech and well...upgrade it, they are made of organic metal, they can be solid and then be a liquid the next second, they can also shot a ray of green plasma from his 'eye'.**

The brown lizard alien was speaking so fast nobody understood him, everyone else had the look of shock displayed, even the dinosaur alien, the bird like alien leaned over to Shepard. "Looks like you're short of one hundred cedits, Shepard." The bird like alien said smugly.

"Fuck you Garrus." Shepard said, though it was in a joking manner as they watched their new ally(?) merge with the terminal and the guns hummed to life, shooting down the Collctor ships.

"Now that is the most beautiful thing I've ever seen...kinda sad really." The hooded woman said as then she saw another bright green flash and the Galvanic Mechamorph was replaced by a plant based alien.

**"Swampfire!!" **The Methanosian called out and Kusumi looked at the alien and smirked.

**Alien Database.**

**Name: Swampfire**

**Species: Methanosian**

**Homeworld: Methanos**

**Abilities: Methanosians are capable of igniting the methan in their palms and throwing fireballs or just a constant stream of fire, and the stench the methan causes is a seconday defense for predators, and they are capable of incredible regeneration and can control plantlife.**

"Do the words 'I am Groot' mean anything to you?" She asked and Swampfire looked over at her, laughing.

"Did you seriously just make a 'Guardians of The Galaxy' reference?" He said between laughs.

Miranda's eyes were practically bulging out of her head, how was this new alien shapeshifting? It seemed to do with that strange symbol on its chest.

Unfortunately, asking the new alien would have to wait because Collectors quickly began firing at them, they all took cover except for Swampfire, who was just regenerating from the wounds.

"Yeah...Gonna need more firepower..." Ben said and tapped the Ultimatrix symbol, four spikes came out of it and in a faint green flash, he became more tree like, blue shells all over the bulky body.

**"Ultimate Swampfire!" **Ben bellowed out before holding out his hands, unleashing massive streams of blue fire, but a Collector ship which had colonists in it began flying away.

"No! They got Donny, Lilith and Judit! You have to stop them!" Commander Shepard then turned to their new alien friend.

"Can you stop it?" He asked and the alien nodded before and green flash blinded everyone and where the bulky alien stood, was now a seventeen year old kid with brown hair, emerald green eyes, blue jeans, black and white shoes and a strange device on his left hand with the same hourglass symbol.

"Or not..." Ben said sadly and looked at everyone.

**"What the fuck?!?!" **Jack yelled, summing uo everyones thoughts at the moment.


	3. Time to meet the crew

**A/N: Alrighty, back here with another chapter! And I plan to go all the way to ME Andromeda with this fanfiction, and when the events of ME3 appear, I'll introduce Alien X! I don't own Ben 10 nor Mass Effect.**

Commander Shepard cursed, looking at Ben as the ship flew away. "Can't you stop it?" He asked the seventeen year old.

Ben shook his head and held up the Ultimatrix. "Out of juice." He responded and Shepard sighed, Garrus walked up and placed a hand on his shoulder.

"Hey, if it wasn't for you, we might not have saved as many as we did Shepard." Garrus said, trying to reassure his friend.

"I've heard that name, some big time Alliance hero right?" The colonist asked.

"You are standing in front of a legend Dolan...and a ghost." A female voice said, the voice belonged to a Ashley Williams.

Dolan scoffed. "Figures you'd stay while hundreds died..." He then walked away, Ben was leaning against the wall, wishing he could've gotten here sooner.

"I'm sorry..." The hero muttered and looked at the Ultimatrix, seeing he had three hours till it recharged, most likely due to the rapid transformations.

**"I can't believe your working for Cerberus now!" **Ashley screamed at Commander Shepard and Garrus before turning around and walking away.

Shepard sighed and turned to the teenage shapeshifter. "I fully expect my questions answered." He said sternly at the teen, who only nodded.

**"ALL LIFE WILL BE ERADICATED." **A lone Harbinger stated, Jack used her biotics to tear it apart...but it got one shot in.

**"Ashley!" **Shepard yelled and ran to his downed friend, Ben and Garrus ran over to try and help Ashley as the Normandy SR-2 landed.

**\- The Normandy SR-2 -**

Ashley was placed inside the medical ward, the squad gave Ben quick glances but didn't ask any questions, that would definitely come later. The squad was situated in the Hanger Bay, where Ben was introduced to the crew, Garrus was the bird-like alien, Grunt was the dinosaur one, Miranda was the woman in the tight body suit, Ben quickly remembered Paradox's words to be patient with her, Jack was the bald tattooed one, Kasumi was the hooded one, Mordin was the lizard looking alien, Jacob was the other man and finally Commander Shepard, who just stepped off the elevator and locked eyes with Ben.

"Everyone get comfortable, I think its time we get some answers." He sat down, directly across from Ben, when he noticed something, he had this look in his eyes, the eyes a soldier would have, a kid his age shouldn't have those eyes.

"Who the hell are you?" Jack asked and Ben looked over at her.

"My name is Ben Tennyson." He answered, when Miranda stepped forward.

"What were those things you were turning into?" She asked, curious as maybe he could be a asset to Cerberus.

"Aliens."

"Bullshit, no such things exist." Miranda scoffed and Ben narrowed his eyes.

"That's where my story gets confusing...They are aliens native to /my/ universe." Ben explained and Garrus' jaw fell open, along with everyone else's jaw, this kid just proved the Multiverse Theory was right!

"How did you get here?" Kasumi asked, tilting her head to the side.

"Time-traveling immortal professor."

"You're pushing your luck kid." Shepard said.

Ben then held up his arms in defense. "Believe it or don't, now...here's my orgin story."

Ben told them about his orgins, gaining the first Omnitrix when he was ten, that made everyone have more respect for him but Miranda was shocked, he gained that power before hitting puberty! He told them some of his adventures, mainly the Hybreed invasion, Vilgax, and Albedo, and then finally getting the current Ultimatrix, and how many transformationshe had at his disposal, which took up some time, he had one hour till he could transform again.

Shepard had hit the jackpot! He grinned and held his hand out to the teen. "Welcome to the crew, Tennyson."

Ben grinned and shook Shepard's hand, that's when Mordin came in, a sad expression on his face. "Shepard..." He said and everyone looked over at the Salarian. "I'm sorry...she's gone..."

Sherpard got up and left, to mourn the loss of his friend, everyone left until it was just Garrus and Ben left. "Hey...Can I get some of this universe's history? I wanna know the lay of the land otherwise I'm just dead weight."

Garrus smiled and nodded. "Yeah, you told us your history, only seems fair I tell you ours."

"Even what Cerberus is?" Ben asked in curiosity, Garrus' expression darkened.

"Especially what Cerberus is." He said in a grim and dark voice.

**\- Two hours later -**

Pissed off couldn't even discribe the level of anger Ben had, the atrocities Cerberus had done pissed Ben off to no end, Ben was currently in the elevator going up to the Captain's Cabin, when it stopped and Miranda stepped on, she pressed the CIC button and looked at Ben.

"I have been instructed to show you to your cabin." She said, noticing his look of anger.

"Yeah, whatever." Ben said with venom.

"I'm sorry, have I done something to offend you?" She asked and Ben turned his head to as the elevator stopped and opened to CIC.

"You? I don't know. Cerberus? Definitely." He said as Miranda stepped off.

"Don't believe everything that you hear Ben. All I ask is that you see our story thoughour perspective." She walked away, cursing mentally as they already managed to turn possibly the most powerful person in the galaxy against her and her organization.

Ben then shook his head as the elevator moved to the Captain's Cabin and he walked in, marveling at the size of the room. _'This is bigger than my whole apartment room!'_ He then say Shepard there and he sat down next to him.

"Figured Garrus wouldn't come up first." He said and looked over at Ben.

"Me and him chatted for a bit in the Hanger Bay, I know the pain you have been through."

"How old are you Ben?" Shepard asked, he guessed he was sixteen or seventeen, maybe eighteen.

"Seventeen." Ben answered without hesitation.

"Yet you have the eyes of a soldier triple that age...But I think I'll be fine for the most part...Just missing my old crew."

Ben knew that voice all too well and he smirked. "What was her name?" Garrus had told Ben who Shepard's original crew was.

Shepard had a nostalgic smile on his face. "Liara T'Soni, she was cute, smart, and didn't know how to act around non-asari..."

He chuckled softly and smiled, patting Shepard on the back. "Alright Robocop, what's our next objective?" The two men shared a laugh as Shepard got up, giving Ben the middle finger.

"We're picking someone else up, bjt first we are gonna get you some armor." He said and Ben nodded and stood up, following Shepard to the elevator.

"Is he or she part of your original crew?" Ben asked as he stood necnext to Shepard, who hit the CIC button, a big nostalgic smile on his face.

"Yup, she's part of my original crew...You'll love her, she's a hopeless romantic. Like you." Shepard said with a small chuckle while Ben's cheeks now had a faint shade of red.

**A/N: Sorry guys, no action in this chapter, Ben just getting acquainted with some of the crew really, and to Anonymous who asked if Ben will be getting Mass Effect aliens, yes, yes he will but they will be my unique twist for Ben's version of them.**

**Now I am gonna walk into my fallout shelter...**

**I'm sorry everyone but i hate Kaiden and Ashley as characters, they tell Shepard to basically fuck off in the 2nd game and try to kiss up to Shepard in the 3rd, but hey, they got Ben to replace Ashley so...**


End file.
